perldebug - Perl Debug

This chapter describes:

• The built-in debugging tool.
• An example debugging session.

Perl Buit-in Debugging Tool

Perl on Linux offers a built-in debugging tool. It is an interact command driven and source line level debugger. To invoke the debugger, you need to run Perl with the debug option: -d. For example, "perl -d MyProgram.pl".

When a Perl program is executed with the debug option, execution will be stopped at the first executable statement and a debug command prompt will be displayed ready to take your debug commands.

Commonly used debug commands:

• h - Printing help information.
• l - Listing next 10 lines of source file.
• n - Executing the program until the next statement in the same code unit. This is called step-over in many other debugging enviroments.
• s - Executing a single statement until the next statement in the same code unit or a subroutine unit called by the current statement. This is called step-into in many other debugging enviroments.
• p exp - Printing the value of the expression: exp, in a scalar context.
• x exp - Printing the value of the expression: exp, in a list context, and in a nicely formatted way.
• b - Setting a breakpoint at the current statement.
• b line - Setting a breakpoint at the statement of the specified line.
• d - Deleting the breakpoint at the current statement.
• d line - Deleting the breakpoint at the statement of the specified line.
• c - Continuing the execution until the next statement with a breakpoint.
• q - Quiting the debugger environment.

Example Debugging Session

Here is the sample program that I used to practice the debugging commands:

#- SubParamList.pl
#- Copyright (c) 1995 by Dr. Herong Yang
#
   $a = 3.14; 
   @a = ('mon','tue','wed','thu','fri');
   $a{'mon'} = 1;
   $a{'tue'} = 2;
   $a{'wed'} = 3;
   &subParamList($a);
   &subParamList(@a);
   &subParamList(%a);
   &subParamList($a,@a,%a);
   exit;
sub subParamList {
   print join(',',@_), "\n";
}

Output of normal execution:

Res 1: 1
Res 2: 0.999999999999
Res 3: 

Let's the built-in debugger to find out why I am getting nothing for "Res 3". Here is a recorded debugging session of mine:

main::(DebugTest.pl:4):    $res = &max(1.0, 0.5, 0.999999999999);
  DB<1> l
4==>       $res = &max(1.0, 0.5, 0.999999999999);
5:         print "Res 1: ", $res, "\n";
6:         $res = &max(0.999999999999);
7:         print "Res 2: ", $res, "\n";
8:         $res = &max();
9:         print "Res 3: ", $res, "\n";
10:        exit;
11      sub max {
12:        my $max = shift(@_);
13:        foreach $val (@_) {
  DB<1> b 8
  DB<2> s
main::max(DebugTest.pl:12):        my $max = shift(@_);
  DB<2> x @_
0  1
1  0.5
2  0.999999999999
  DB<3> b
  DB<3> c
Res 1: 1
main::max(DebugTest.pl:12):        my $max = shift(@_);
  DB<3> x @_
0  0.999999999999
  DB<4> c
Res 2: 0.999999999999
main::(DebugTest.pl:8):    $res = &max();
  DB<4> s
main::max(DebugTest.pl:12):        my $max = shift(@_);
  DB<4> x @_
  empty array
  DB<5> n
main::max(DebugTest.pl:13):        foreach $val (@_) {
  DB<5> x $max
0  undef
  DB<6> c
Res 3:
Debugged program terminated.  Use q to quit or R to restart,
  use O inhibit_exit to avoid stopping after program termination,
  h q, h R or h O to get additional info.
  DB<6> q

Can you see why I am getting nothing for "Res 3"?